The present invention relates to a magnetic bearing characterized in having a rotor levitated by electromagnet power and maintained at a certain levitated constant position through detecting its position.
Recently electromagnetic levitation devices which can convey or carry works without contact are utilized for carrier system in such an atmosphere where highly clean conditions must be maintained, like IC manufacturing equipments. In such electromagnetic levitation devices, a magnetic bearing having a rotor which is levitated with electromagnetic power and of which the levitating position is maintained in a certain constant position has been used. FIG. 14 shows a block diagram of a conventional electromagnetic bearing configuration. This kind of electromagnetic bearing has two electromagnets 42a, 42b which are positioned to be facing each other, and two position sensors 43a, 43b. Moreover, the electromagnetic bearing comprises a bridge circuit 44 to which a detecting signal from position sensors 43a, 43b is input, a comparator 46 which compares the output signal from the above bridge circuit 44 with a standard signal 45, a signal processing circuit 47 for processing an output signal of the comparator, and amplifier circuits 48a, 48b that amplify the output signal from the signal processing circuit 47 and transfer its output to electromagnets 42a, 42b.
In this electromagnetic bearing, a rotor 41 is levitated and held in a predetermined position. The position of the rotor is detected by the position sensor 43a, 43b, and a signal that changes according to the position of the rotor 41 is output from the bridge circuit 44 to the comparator 46. By operation the comparator 46, is obtained a signal that changes according to the compared difference of the output signal from the bridge circuit 44 with the standard signal 45, that is, a signal that changes according to the compared difference of the position of the rotor with its standard position, and this signal is then processed by signal processing circuit 47, and then output to the amplifiers 48a, 48b. By the amplifiers 48a, 48b the exciting current is supplied to electromagnets 42a, 42b. The rotor 41 is controlled to be held at a predetermined position as above.
In the prior art electromagnetic bearing, it is technologically difficult to detect the rotor's position with high accuracy using a conventional position sensor and because a position sensor is needed to detect the rotor position there exists a problem in that an expensive sensor of higher accuracy is needed in order to accurately control the rotor position.
And in the prior art electromagnetic levitation system, it is usually the case for holding the position accurately that an integrating element is inserted into the feedback loop. But this kind of an integrating element is more or less an approximate one. For example, it is not possible that direct current gain can not be infinite due to the step output of the operational amplifier when the integrating element is comprised of an analog circuit. And a further deviation is included to a certain degree because of quantization errors in AD conversion and in computer process when the integrating element is comprised by a digital control device. These problems show the existence of a certain limitation in the improvement of sustaining accuracy by arrangement of integrating element in the prior art electromagnetic levitation systems.